Resin-based dental composites have been widely used in dentistry to restore carious teeth. But composites may have shorter life than amalgams. The two leading causes for the failure of dental composite restorations in clinic are secondary (recurrent) caries and bulk fracture. To reduce secondary caries, research efforts have been directed towards the development of the dental composites that release anti-caries agents (F-, Ca2+ and PO4 3- ions).Current fluoride-releasing dental composites have very low fluoride-releasing and recharge capability. Glass ionomers and resin-modified glass ionomers (RMGI) have high fluoride release but they have low strength and fracture toughness. Despite significant development in this area in past 20 years, dental composites reinforced with particulate fillers, particularly those releasing anti-caries agents, still have inadequate strength and fracture toughness. To overcome the fracture problem, glass fibers and ceramic (SiC and Si3N4) whiskers have been used to improve the strength and fracture toughness of composites, but the chemical stability, esthetics and handling properties of these materials are unsatisfactory. The goal of this in vitro exploratory project is to study the feasibility of using zirconia-based ceramic nanofibers to reinforce fluoride-releasing dental composites. The hypotheses are: (1) zirconia-based ceramic nanofibers (100-300 nm) will significantly increase the mechanical properties and fracture toughness of the composite and provide better aesthetics and handling properties than the coarse fibers;(2) the composite reinforced by zirconia-based ceramic nanofibers will have significantly better chemical stability than glass fibers or Si3N4 whisker-reinforced composites. The aims of this project includes: (1) fabricating nanofibers of zirconia-based ceramics using sol-gel and reactive electrospinning process;(2) improving nanofiber dispersion and nanofiber-resin matrix interfacial properties by treatment with special surface coupling agents;(3) fabricating the zirconia nanofiber-reinforced, fluoride-releasing composites dental composites and testing their flexure strength, fracture toughness, wear resistance, color and gloss indices, and chemical stability after ageing in water. We anticipate that the novel fluoride-releasing composite reinforced by zirconia nanofibers will have significantly improved mechanical properties as well as high fluoride-releasing and recharging capabilities. The long-term goal of this project is to develop a series of nanofiber-reinforced dental restorative materials that have anti-caries efficacy, good aesthetics and handling properties as well as good mechanical properties. These improved dental materials will reduce secondary caries and prolong the life of restorations.